1. Field of the Invention
The invention relates to an illumination mask for a device for the range-resolved determination of scattered light, having one or more scattered-light measuring structures, which respectively include an inner dark-field zone which defines a minimum scattering range, to a corresponding device which comprises the illumination mask for the provision of measuring radiation on an entry side of a specimen and a detection part for the range-resolved detection of scattered light on an exit side of the specimen, to an image-field mask which can be used for this, to an associated operating method and to a microlithography projection-exposure system having such a device.
2. Description of the Related Art
Such devices and methods are used, for example, for the purpose of drawing from the scattered light of optical components detected in a range-resolved fashion conclusions about their optical properties and optical quality, for example with regard to surface roughnesses, contaminations and material inhomogeneities. Moreover, a scattered-light portion thus determined can correctively be considered if a spatially resolved knowledge, which is as accurate as possible, of the quantity of radiation actually supplied by an optical system is desired in corresponding optical applications, such as lithographic exposure apparatuses, for example. In the text which follows, the term light is used for the sake of simplicity to denote electromagnetic radiation of arbitrary wavelength, in particular radiation in the UV or EUV range.
For the purpose of range-resolved determination of scattered light, in a conventional method a non-transparent object which acts as dark-field zone is introduced into the beam path of a directed illuminating radiation having a larger beam cross section compared to the object, such that a shadow image of the object is produced. If the system to be examined, presently also termed a test component, is an imaging optical system, the non-transparent object is positioned in the object plane of the system. The object is imaged by the imaging system onto an image of the object in the image plane which may be enlarged or reduced by the reproduction scale and which is denoted as an “aerial image” below, as is also denoted the pure shadow image in the case of test components which are not focusing imaging systems.
Scattering of the illuminating light by the test component has the effect that scattered light passes laterally into the dark zone of the surface of the aerial image. The intensity of this scattered light normally decreases with increasing distance from the edge to the middle of the aerial image. Depending on the lateral distance from the edge of the aerial image, denoted below as scattering distance or scattering range, an intensity distribution of the scattered light thus arises which is determined as a function of distance, that is to say range-resolved.
In a conventional mode of procedure, a sensor surface is introduced as a component of a detection part into the plane in which the aerial image is produced or detected. For the sake of simplicity, the term image plane is also used for this plane whenever the test component is not a focusing imaging system. It is important here that the sensor surface is smaller than the aerial image so that a minimum, lateral scattering distance remains between the edge of the sensor surface and the edge of the aerial image. In consecutive measuring operations, objects of different size are then positioned such that the size of the aerial image, and thus the minimum scattering distance, changes. It is known for this purpose to provide objects of variable size at a distance from one another on an otherwise transparent illumination mask. Chromium squares of variable edge length, for example, are used as objects. The sensor surface correspondingly has a quadratic geometry. In order to vary the minimum scattering distance, the illumination mask is displaced such that the objects applied to it are brought one after the other into the beam path and imaged onto the sensor surface centred on the image side in the plane of the aerial image.
In one of the conventional measuring methods, the entire intensity of the scattered light impinging on the sensor surface over a fixed time interval is determined integrally for each object size, for example with the aid of an electro-optical measuring element. Each object size is assigned a minimum scattering distance in the image plane, the maximum scattering distance being infinity in theory for all objects. The scattered light distribution can be reconstructed from the intensity distribution as a function of the minimum scattering distance.
Instead of directly measuring the intensity of the scattered light on the sensor surface, in the case of an alternative conventional measuring method a photo-resist layer is provided in the image plane as sensor surface, and in different measuring operations a respectively different region of the photo-resist layer is irradiated with illuminating radiation of increasing intensity. Then, that limiting value for the intensity is determined in the case of which the aerial image first vanishes entirely as a structure in the photo-resist layer. This limiting value for the intensity of the illuminating light is used instead of the light intensity integrated over the sensor surface in order to determine the scattered light distribution. These and further details relating to conventional methods of determining scattered light are to be found in the relevant literature, see, for example, the magazine article by J. P. Kirk, “Scattered Light in Photolithographic Lenses”, SPIE, Volume 2197 (1994), pages 566-572, and the magazine article by Eugene L. Church, “Fractal Surface Finish”, Applied Optics, Volume 27, No. 8 (1988), pages 1518-1526.
In the case of the conventional methods outlined above, effects with a long scattering range are normally superimposed on effects of short to medium scattering ranges. The non-transparent objects on the illumination mask certainly define a minimum scattering range, but the maximum scattering range is limited only by the size of the object field, that is to say the illumination mask. Moreover, since the squares have a relatively large spacing from one another, in order not to disturb one another during the measurements, and are typically arranged in a nonsymmetrical distribution, the long-range scattering light intensity is a function of the field point, and so systematic measuring errors can occur.
The devices and methods currently of interest for determining scattered light are chiefly used in the field of medium scattering ranges, what corresponds, e.g., to typical object-side scattering ranges in a region from approximately 4 μm to approximately 1000 μm and/or image-side scattering ranges from approximately 1 μm to approximately 250 μm.
Especially for the characterization of optical systems, and in particular optical imaging systems, not only the scattered-light behavior thereof but also the behavior thereof with respect to imaging defects are of interest. Various wavefront-measurement devices and methods have been proposed for this, which can be used to find the effect of the specimen on the wavefront behavior, for example by an interferometric or Moiré fringe technique, from which the imaging defect behavior can be deduced. An important field of application is the wavefront measurement of optical systems for microlithography, in particular projection objectives for microlithography projection-exposure systems for the patterning of semiconductor wafers.
Imaging defects of such high-resolution imaging objectives can be determined with the required accuracy by using the wavefront-measurement technique, in which case radiation of the same wavelength as in the useful imaging operation of the objective is preferably used for the measurement, and it has recently become more common to use UV radiation in the wavelength range shorter than about 200 nm. Interferometric measurement devices which work in this way with measuring radiation at the operating wavelength of the specimen may also be referred to as operational interferometers (OIF) for this reason and they may, for example, be integrated into the projection-exposure system in question. Various types of interferometric wavefront-measurement devices are currently used for this purpose, such as those which are based on shear interferometry or point diffraction interferometry, or those of the Ronchi type, Twyman-Green type or Shack-Hartmann type.
As an alternative to single-channel measurement devices, it is also possible to use multi-channel measurement devices with which measurements can be taken from a plurality of field points in parallel, i.e. simultaneously, so that shorter measuring times for full-field measurements can be achieved compared with single-channel devices. For example, the laid-open patent specification WO 01/63233 A2 (corresponding to U.S. Pat. No. 2002/0001088) discloses a multi-channel measurement device which works with lateral shear interferometry. Such a device typically includes an illumination mask, which is to be arranged on the object side of the specimen and has a structure for generating wavefronts separately for the various field points, and a diffraction-grating structure to be arranged on the image side of the specimen for the various field points. With a suitable design of the system, the intet-ferograms for the individual field points can be kept substantially separate and can be discriminately recorded by a downstream detector, with a detector surface thereof preferably being placed at a very short distance from the diffraction-grating structure.